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Bubble-free replication of large area microstructures using gas-assisted UV embossing with modified reversal imprinting and gap-retained vacuuming
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Image of FIG. 1.
FIG. 1.

(Color online) Conventional UV embossing process with possible mechanism of air bubbles formation under vacuum.

Image of FIG. 2.
FIG. 2.

(Color online) Difficulties encountered while removing air bubbles using vacuum in the conventional UV imprinting process.

Image of FIG. 3.
FIG. 3.

SEM micrographs showing air bubble defects observed in large area conventional UV embossing process. In the upper magnified inset, the defect is caused by unevenness between the substrate and the stamper, and the lower magnified inset, defects are typical air bubbles.

Image of FIG. 4.
FIG. 4.

(Color online) When the evacuation stage is performed in vacuum, a gap between the substrate and resin-coated stamper can be retained during the large area UV embossing process combined with modified reversal imprinting. The air bubbles contained in the liquid UV-curable resin can be easily extracted from the open surface of the liquid.

Image of FIG. 5.
FIG. 5.

(Color online) Figures showing the apparatus and stages of the gas-assisted UV-curing embossing with modified reversal imprinting setup and a spring-activated gap-retained substrate holder. (a) The preparation and (b) the upper and lower chambers are closed and then vacuumed. The substrate is separated from the resin-coated stamper with the gap-retained substrate holder. (c) Nitrogen gas is pumped into the upper chamber to provide the embossing pressure. Subsequently, UV-curable resin is cured under UV-LED irradiation, and the microstructures are transferred from the stamper onto the PMMA substrate. (d) Nitrogen gas pressure is released and then both chambers are opened. The PMMA substrate with large area microstructures is obtained.

Image of FIG. 6.
FIG. 6.

(Color online) Photograph of the large area stainless steel stamper used in this study. On the stamper, there are microstructures of a microlens array with an average sag height of 8.0 m and an average diameter of 120.0 m, which was fabricated by photolithography and wet etching.

Image of FIG. 7.
FIG. 7.

(Color online) Experimental results of the gas-assisted UV-curing embossing with the modified reversal imprinting technique and gap-retained vacuum mechanism. (a) The SEM image of the fabricated microstructures (microlens array) on the PMMA substrate. Extremely trifling air bubble defects have been detected over the whole area of the PMMA substrate after the proposed process. (b) The surface profile of the fabricated microstructure (microlens array). The average diameter and sag height of microlens are 120.0 m and 8.0 m, respectively.

Image of FIG. 8.
FIG. 8.

(Color online) PMMA substrate is divided into 16 (4 × 4) sub-areas. The optical microscope image of a region is randomly selected from each sub-area. There are no air bubble defects observed, so that the modified reversal imprinting technique combined with gap-retained vacuum mechanism is effective in removing air bubble defects.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Bubble-free replication of large area microstructures using gas-assisted UV embossing with modified reversal imprinting and gap-retained vacuuming